The invention relates to an electroluminescent device comprising a first electrode, a second electrode and an ionic organic layer which is in contact with said second electrode, which layer contains a conjugated compound and a salt, at least an anion or cation of said salt being mobile relative to the second electrode.
The invention also relates to a method of manufacturing such an electroluminescent device.
An electroluminescent (EL) device is a device which, utilizing the phenomenon referred to as electroluminescence, emits light when the device is suitably connected to an electric power source. If the light emission originates in an organic material, the device in question is referred to as an organic electroluminescent device.
An organic EL device can be used, inter alia, as a thin light source having a large light-emitting surface, such as a backlight for a liquid-crystal display or an illuminated logo. An organic EL device can also be used as a "display" if the EL device comprises a number of EL elements, which are or are not independently addressable. Integrated EL elements can be obtained, for example, by providing both sides of an electroluminescent layer with a patterned electrode layer.
The EL device is provided with at least two electrodes which are in contact with the organic EL layer. By applying a suitable voltage, the negative electrode, i.e. the cathode, will inject electrons and the positive electrode, i.e. the anode, will inject holes. The cathode and the anode may be coplanar, however, it is more customary to arrange them one above the other in such a way that the EL layer is sandwiched between them. In the latter case, at least one of the electrodes should be transparent to the light to be emitted. A known transparent electrode material for the anode is, for example, indium tin oxide (ITO). Apart from the EL layer, known organic EL devices generally comprise additional organic layers to enhance the charge injection and/or charge transport.
An EL device of the type mentioned in the opening paragraph is described in the non-prepublished Dutch Patent Application NL 1002944 published as international application WO 97/40648 Oct. 30, 1997 (corresponding to U.S. application Ser. No. 08/842,525, filed Apr. 24, 1997), which is in the name of the current Applicant. In said Application, a description is given of an EL device which is composed of a stack of layers comprising, in succession, an ITO electrode layer, an ionic organic layer and an indium electrode layer. The ionic layer contains a poly(phenylene vinylene) to which a quaternary amino group is covalently bonded. The counterion of the quaternary amino group is mobile. By virtue of the presence of mobile ions, which can move relative to the electrodes under the influence of an electric field, the voltage necessary to attain light emission is much lower than the voltage necessary for light emission in a corresponding EL device which does not contain such mobile ions.
The ionic organic layer containing the quaternary amine can be obtained by exposing a precursor layer containing the corresponding tertiary amine to an alkylating agent. As regards the other process steps in the manufacture of the EL device, said Application teaches that techniques which are known per se are suitable. Said Patent Application does not state how a patterned light-emitting surface is to be manufactured. According to said Patent Application, individual EL elements can be obtained by using a structured electrode layer. However, it is necessary to avoid the formation of EL elements at locations where they are undesirable, as would be the case, for example, if an electric power supply line of a positive electrode of an EL element overlaps an electric power supply line of a negative electrode of another EL element, while they are both energized at the same time. The necessity of avoiding such undesirable EL elements restricts the designer's freedom to realize a desirable pattern.